It has long been recognized that the fluorination of a polymeric surface can increase the water-wettability of the surface under certain conditions L. J. Hayes, J. Fluorine Chem., 8, 69 (1976). It is also known that aliphatic fluorination, rather than aromatic fluorination, is the primary reaction L. J. Hayes et al., J. Fluorine Chem , 10, 1 (1977) In addition, monofluorination of a carbon atom in a polymer is preferred over difluorination since excessive difluorination may decrease the adhesive properties of the polymeric surface.
Several methods have been disclosed for treating polymeric surfaces with fluorine-containing gases to promote the adhesion of subsequently applied coatings. Treatment times are generally in the range of several minutes to several hours, and fluorine concentrations are at least 1 percent fluorine by volume. In fact, according to the prior methods, fluorine concentrations are usually much greater than 1 percent by volume.
Those methods that disclose relatively short treatment times employ fluorine-containing gases having relatively high fluorine concentrations. On the other hand, those methods that disclose relatively long treatment times are able to employ gases having relatively low fluorine concentrations.
The methods of the prior art, however, do not describe the simultaneous use of both relatively short treatment times and relatively low fluorine concentrations to achieve the improved adhesion of coatings to a polymeric surface. This is probably because such superficial treatment does not perceptibly modify the adhesion characteristics of ordinary coatings. The economic advantages of short treatment times in combination with low fluorine concentrations will be apparent to those familiar with commercial processes of the type described herein.
With reference to those prior methods, U.S. Pat. No. 4,237,156 to Boultinghouse describes a process for treating a surface with a fluorine-containing gas to render the surface water-wettable and receptive to an adhesive, a coating or the like. The disclosed surfaces are formed of poly(arylene) sulfide resins. Treatment times range from about 30 seconds to greater than 30 minutes, and the concentration of fluorine in the gas is within the broad range of about 1 percent to 99 percent by volume. The disclosed reaction temperature is between about 65 degrees C. and 100 degrees C. In the examples of Boultinghouse, the following conditions were employed a treatment time of 5 minutes, a fluorine concentration of 10 percent by volume and a temperature of 82 degrees C.
U.S. Pat. No. 4,296,151, also to Boultinghouse, relates to the treatment of a surface with a fluorine-containing gas to achieve the same effects described in the '156 patent. The range of treatment times and the concentration of fluorine in the gas is similar to that described above; but the permissible range of the reaction temperature is broadened to from about 15 degrees C. to about 100 degrees C.
With respect to a continuous process (presumably for commercial use), however, Boultinghouse states at column 3, lines 29-34, of the '151 patent that the fluorine concentration of a fluorine-containing nitrogen gas should be at least 10 percent by volume fluorine and that the fluorine concentration of a fluorine-containing helium gas should be at least 75 percent by volume fluorine.
In the examples of the '151 patent, Boultinghouse describes a treatment time of 5 minutes, a fluorine concentration of 10 percent by volume fluorine in nitrogen and reaction temperatures of 82 degrees C. and 93 degrees C.
U.S. Pat. No. 4,454,219 to Yamadera et al. discloses that the ultraviolet cure of a coating may be improved by use of an organic halogen compound in the coating composition. The disclosed, organic halogen compounds include carbon tetrachloride, chloroform, bromoform, iodoform and the like. The preferred organic halogen compounds include those having a tribromomethyl group.
Yamadera et al. suggest that halogen radicals produced directly or indirectly by irradiation of the organic halogen compound withdraw hydrogen from a hydrogen donor, and the resulting hydrogen halide reacts with a aliphatic amino group-containing monomeric unit of a polymer to form polar ionic groups, in particular, quaternary ammonium salts. The polar ionic groups provide improved adhesion of the coating to a surface to which the coating is applied.
Yamadera et al., however, do not disclose the use of fluorine. Moreover, the disclosure of Yamadera et al. relates to a material that is incorporated into the coating composition and not to any moieties that might be present on the surface being coated.
U.S. Pat. No. 4,491,653 to McGinniss et al. relates to a method for fluorinating the surface of a polymeric solid which comprises contacting the solid with a dilute fluorine gas at about room temperature to partially fluorinate the solid surface. The contact time is from about 1-30 minutes, and the concentration of fluorine in the gas is less than about 4 percent by volume. McGinniss et al. refer to their method as an extra dilute phase fluorination reaction, and yet in the disclosed examples the solid was contacted with a gas having a fluorine concentration of about 2.55 percent by volume.
Moreover, McGinniss et al. indicate at column 2, lines 11-12, that with respect to methods for fluorinating the surface of a polymeric solid, it is difficult to predict the resulting properties of the treated material.
Upon review of the prior art, one would not expect a fluorine-containing gas containing less than 1 percent flourine by volume to have any significant effect on the adhesion properties of a polymeric surface, particularly when the surface is treated with the gas at relatively short exposure times of less than about 1 minute. Indeed, for ordinary heat-cured coatings no improvement in adhesion has been observed. Accordingly, the improved adhesion of a radiation-curable coating composition to the polymeric surface of a magnetic recording structure, such as a tape or disk, when treated with a fluorine-containing gas, in the superficial manner described herein, is unexpected.